A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of one, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
European Patent Application No. 1 491 963 and U.S. Patent Application Publication No. 2002084428 disclose a lithographic apparatus that comprises a contamination trap, also called “foil-trap”. The foil trap is placed in the path of the radiation beam and contains a plurality of strips (also called lamellas), which are positioned so that the radiation beam is able to pass through spaces between the strips. Typically, the strips are arranged as spokes on a hub that is located on the central axis of the radiation beam. The strips of the foil trap serve to trap unwanted matter that would otherwise cause damage to the optical elements (typically mirrors) that are used to condition the beam for projection of the pattern onto the substrate. The sequentially first optical element, the collector mirror that gathers the radiation from the plasma source is particularly at risk.
A preferred EUV radiation source is a plasma source. A major problem with this type of radiation source is that accelerated particles can be launched from the source into the path of the beam. Fast plasma or charged debris particles can be launched into the path of the beam, for example. Particles or ions/neutrals or plasma may include working material of the source, such as Sn, Xe, . . . or material from area's close to the emitting region such as Mo, W, and/or Fe.
As described in EP 1 491 963 and US2002084428, a foil trap between the plasma source and the collector mirror addresses part of this problem. The foil trap intercepts debris that does not travel in parallel with the beam. Moreover, debris that travels in parallel with the beam can be trapped by introducing gas in into the foil trap. EP 1 491 963 describes an embodiment wherein the strips of the foil trap are charged to mutually different voltages so as to create electric fields between the strips transverse to the direction of the radiation beam. This is done to trap fast charged particles, such as Xe ions that emerge from the plasma source. As described in this document, the electric field between the strips applies to the particles that travel through the foil trap, giving them an azimuthal velocity component towards the strips.
Reliance on this effect tacitly assumes that no plasma exists between the lamella, because a plasma would result in a spatial charge distribution that counteracts the external electric field from the strips. Therefore, the electrically charged foil trap of EP 1 491 963 and US2002084428 relies on the fact that density of charged particles is too low to create plasma inside of a foil trap.